  // fill the vector with values
  void cmd_fill(istringstream& ss) {
    string what;
    if(!(ss>>what)) throw "syntax: fill con|cap|source";
    if(what=="source") fd[current]->solver->SetSource(fd[current]->src);
    else if(what=="con") fd[current]->solver->SetConductivity(fd[current]->con);
    else if(what=="cap") fd[current]->solver->SetCapacity(fd[current]->cap);
    else throw "unknown fill option";
  }
